Conventional digital data storage (DDS) format devices provide reliable storage and retrieval of large amounts of digital data. Such devices are defined in ISO/IEEC Standard 10777:1991E.
In a DDS read/write mechanism, as defined in the ISO/IEEC Standard 10777:1991E, data is recorded on an elongate tape data storage medium coated with a magnetic coating. A rotating drum carries a plurality of read heads and a plurality of write heads. The elongate band of magnetic tape is passed around the rotating drum, resulting in a plurality of physical data tracks written in parallel across the elongate band of magnetic tape between opposite edges of the tape.
Referring to FIG. 1 herein, there is shown schematically a layout of a tape data storage cartridge in relation to a tape drive mechanism according to the known DDS-1 to DDS-4 formats, in which an elongate band of tape is contained within a removable tape cartridge 100. The tape cartridge is inserted between a pair of guides of a tape drive mechanism to locate the cartridge in the mechanism. A rotating read/write head 101 comprises first and second read heads and first and second write heads situated at substantially equidistant points around a circumference of the rotating head. The head rotates on top of a substantially cylindrical metallic plinth 102. A main central axis of a cylinder formed by the outer surfaces of the drum and the plinth is directed offset from a line normal to a plane of a base plate 103, so that the effect is that as the band of tape traverses around part of the circumference of the cylindrical head plinth, the rotating heads describe a path diagonally across a width of the tape in successive passes of the heads past the tape. The read/write head rotates at a speed of approximately 11,400 revs per minute.
Referring to FIG. 2 herein there is shown schematically a tape path of the elongate magnetic tape data storage medium 201 as it is drawn past the rotating drum containing the read and write heads. The tape data storage medium 201 is wound onto a feed reel 202 and a take up reel 203 which are within the removable tape cartridge 100. During normal operation, the magnetic tape 201 is wound from the feed-reel 202 on to the take-up reel 203. The path of the magnetic tape 201 is constrained by a plurality of rollers and tape guides 204-208. Additional tape guides 104, 105 determine the relative positions of the rotating drum 101, the read and write heads 210-213 and the tape data storage medium 201. The feed reel 202 and tape up reel 203 are driven by electric motors to maintain a correct tension in the magnetic tape 201 past the head.
Referring to FIG. 3 herein, there is illustrated schematically the orientation of the magnetic tape 201 with respect to the rotating drum 202. The tape 201 is drawn past the rotating head at a relatively slow tape speed of the order of a few centimeters per second. However, the rotating drum 101 on which the read and write heads are mounted, typically rotates at a few thousand revolutions per minute, so the relative speed of the read and write heads to the drum is of magnitudes of order greater than the absolute tape speed. During a write operation, the write heads record a sequence of tracks diagonally across the elongate magnetic tape 201. The width of such tracks is typically of the order of 6.8 μm. A plurality of data stripes 300 are written in parallel to each other across the width of the tape by the rotating write heads. In a read operation, the read heads trace along the plurality of stripes to read individual physical tracks.
The prior art formats DDS-1, DDS-2 and DDS-3 used an exclusive (XOR) based check sum. However, this was ineffective because a C2 mis-correction will often always give the same XOR based check sum as correct data.
The prior art DDS-4 format used a more effective arithmetic sum. However, this arithmetic sum has a significant chance of giving the same check sum if a mis-correction occurs.
Specific implementations according to the present invention aim to provide a new check sum which is fast, simple to implement and offers the theoretical maximum error correction detection performance which one would expect for a two byte check sum.